In the mining industry, mined ores and coal are upgraded using appropriate separation method. They are usually crushed and/or pulverized to detach (or liberate) the valuable components from waste rocks prior to subjecting them to appropriate solid-solid separation methods. Although coal is not usually pulverized as finely as ores, a significant portion of a crushed coal is present as fines. Froth flotation is the most widely used method of separating the valuables from valueless present in the fines. In this process, the fine particles are dispersed in water and small air bubbles are introduced to the slurry, so that hydrophobic particles are selectively collected on the surface of the air bubbles and exit the slurry while hydrophilic particles are left behind.
A small dose of surfactants, known as collectors, are usually added to the aqueous slurry to render one type (or group) of particles hydrophobic, leaving others unaffected. For the case of processing high-rank coals, no collectors are necessary as the coal is naturally hydrophobic. When the coal particles are not sufficiently hydrophobic, however, hydrocarbon oils such as diesel oil or kerosene are added to enhance their hydrophobicity.
It has been shown recently that air bubbles are hydrophobic (Yoon and Aksoy, J. Colloid and Interface Science, vol. 211, pp. 1-10, 1999). It is believed, therefore, that air bubbles and hydrophobic particles are attracted to each other by hydrophobic interaction.
The floated products, which are usually the valuables, are in the form of aqueous slurry, typically in the range of 10 to 35% solids. They are dewatered frequently by filtration prior to further processing or shipping to consumers. The process of dewatering is often described by means of the Laplace equation:
                                          Δ            ⁢                                                  ⁢            p                    =                                    2              ⁢                                                          ⁢                              γ                23                            ⁢              cos              ⁢                                                          ⁢              θ                        r                          ,                            [        1        ]            
in which r is the average radius of the capillaries formed in between the particles that make up a filter cake, Δp the pressure of the water inside the capillaries, γ23 the surface tension at the water(3)-air(2) interface and θ is the contact angle of the particles constituting the filter cake. The capillary water can be removed when the pressure drop applied across the cake during the process of filtration exceeds Δp. Thus, a decrease in γ23 and θ, and an increase in r should help decrease Δp and thereby facilitate the process of dewatering.
The U.S. Pat. No. 5,670,056 disclosed a method of using hydrophobizing agents that can increase the contact angle (θ) above 65° and, thereby, facilitate dewatering processes. Mono-unsaturated fatty esters, fatty esters whose hydruphile-lipophile balance (HLB) numbers are less than 10, and water-soluble polymethylhydrosiloxanes were used as hydrophobizing agents. More recently, a series of U.S. patents have been applied for to disclose the methods of using a group of nonionic surfactants with HLB numbers in the range of 1 to 15 (Ser. No. 09/368,945), naturally occurring lipids (Ser. No. 09/326,330), and modified lipids (Ser. No. 09/527,186) to increase θ beyond the level that can normally be achieved using flotation collectors and, hence, improve dewatering.
Ever since the flotation technology was introduced to the mining industry, its practitioners have been seeking for appropriate collectors that can increase θ as much as possible without causing unwanted minerals inadvertently hydrophobic. A theoretical model developed by Mao and Yoon (International Journal of Mineral Processing, vol. 50, pp. 171-181, 1996) showed that an increase θ can increase the rate at which air bubbles can collect hydrophobic particles.